The present invention relates to magnetrons for generating high-frequency microwave radiation. The invention is particularly useful in relativistic magnetrons, and is therefore described below particularly with respect to this application.
The relativistic magnetron is one of the most successful classes of high power microwave generators in present use, i.e., generators which are capable of generating electromagnetic power pulses above about 100 megawatts and up to tens of gigawatts. Typical pulse lengths are between a few tens of nanoseconds and a few microseconds.
The relativistic magnetrons produce high power levels with good efficiency and frequency stability. They are basically very similar to the conventional magnetrons developed during World War II.
Both the conventional magnetron and relativistic magnetron include a cathode and an anode in a vacuum chamber radially spaced from each other to define an interaction region, means for producing an electric field E between the anode and cathode, and means for producing a magnetic field B perpendicular to the electric field E. Electrons emitted from the cathode are accelerated by the electric field E towards the anode in the presence of the magnetic field B perpendicular to the electric field to produce the well known E.times.B drift velocity. Within the anode, a set of identical cavities create a slow wave structure. When the phase velocity of the electromagnetic wave rotating in the interaction region equals the E.times.B drift velocity of the electrons, energy is transferred from the electrons to the electromagnetic wave.
The relativistic magnetron differs from the conventional magnetron in the following two basic ways:
(a) The driving voltages and currents in the relativistic magnetron are at least an order of magnitude larger than in the conventional magnetron; thus, the name "relativistic" indicates that at these voltages the electrons gain kinetic energy comparable to the rest mass of the electron (511 KeV).
(b) In the conventional magnetron, the electrons are emitted from a hot cathode. The relativistic magnetron, however, exploits the .extremely high electric field to emit electrons from a cold cathode. The mechanism of action of the cathode is quite complicated and is sometimes known as "explosive emission". Very large current densities are produced, which enable the generation of very high power microwaves.
Since relativistic magnetrons are driven by pulses of several hundred kV, any curved surface at the cathode potential tends to emit electrons and/or to initiate a high-voltage breakdown. In addition, special problems are involved in providing the required high level magnetic field (in the order of a few kGauss), which is a major problem in reducing the size, weight and cost of the relativistic magnetron. Further, a well know problem of relativistic magnetrons of the conventional design is the "axial current", resulting from the drift along the magnetic field lines of electrons emitted from the cathode and leaving the interaction region. These electrons do not contribute to the generation of the microwaves; their energy is lost to heat, and the efficiency of the magnetron is thereby reduced.